Oxomemazine

Oxomemazine is a phenothiazine derivative classified as a first-generation H1-antihistamine, primarily used to treat allergic conditions such as hypersensitivity reactions and pruritic skin disorders.^[1] It is also marketed for cough associated with upper respiratory conditions, often in combination with expectorants, though a 2024 systematic review found no evidence of its clinical efficacy for cough treatment.^[2][3] It competitively antagonizes histamine at H1 receptors to inhibit allergic responses, while also possessing notable sedative, anticholinergic, and antiserotoninergic properties.^[1][4] Developed as part of the phenothiazine class, oxomemazine demonstrates selectivity for M1 muscarinic receptors, enhancing its anticholinergic activity compared to some other antihistamines.^[5] Clinically, it is administered orally in divided doses, with typical adult regimens ranging from 5 to 13 mg daily for allergic conditions and adjusted lower doses for pediatric use based on age.^[1][4] Common adverse effects include drowsiness, dry mouth, vertigo, and gastrointestinal disturbances, with rarer risks such as urinary retention or photosensitivity necessitating caution in patients with conditions like glaucoma or prostatic hypertrophy.^[1][4] Its chemical formula is C₁₈H₂₂N₂O₂S, and it is available in forms such as syrups often combined with expectorants like guaifenesin for symptomatic relief of colds.^[2]

Medical uses

Indications

Oxomemazine is primarily indicated for the symptomatic treatment of dry cough and cough irritations in adults and children over 2 years of age.^[2][6] As a first-generation antihistamine, it provides relief from associated allergic symptoms such as sneezing, runny nose, and itchy eyes in conditions like allergic rhinitis.^[7] It is also approved for the management of urticaria and pruritus, offering symptomatic relief in these allergic manifestations.^[2][4] In secondary applications, oxomemazine alleviates allergic skin reactions and generalized itching, particularly when these are triggered by environmental allergens.^[4] It is occasionally formulated in combination with guaifenesin to address productive cough in the context of upper respiratory tract infections or common cold symptoms, where the expectorant properties of guaifenesin complement its antitussive effects.^[2][8] The evidence supporting oxomemazine's efficacy for cough suppression is limited, with a 2024 systematic review and meta-analysis of randomized controlled trials concluding a lack of robust placebo-controlled data to confirm clinical benefits.^[3] Despite this, its use persists based on historical precedent, having been introduced in Europe for respiratory allergies and cough relief since the 1960s, as reflected in early authorization records from 1964.^[9][10] Oxomemazine is most commonly available as an oral syrup at a concentration of 0.33 mg/mL, administered alone for dry, irritative cough or in multi-ingredient formulations for broader symptomatic relief in allergic or respiratory conditions.^[11][12]

Administration and dosage

Oxomemazine is administered orally as a syrup formulation, typically at a concentration of 0.33 mg/mL, using a provided measuring cup for accurate dosing.^[2][13] It may be taken with or without food, and the bottle should be shaken well before each use to ensure uniform distribution of the active ingredient.^[14] Doses should be spaced at least 4 hours apart, with the initial dose preferably taken in the evening to minimize daytime sedation.^[13] For adults, the standard dosage is 5-13 mg per day, administered in divided doses of 2-4 times daily.^[1] For children aged 2-6 years, the dosage is 2.5-5 mg per day in 2-3 divided doses; for children aged 6-12 years, it is 5-10 mg per day in 2-4 divided doses.^[13] These correspond to volumes of approximately 7.5-15 mL per day for younger children and 15-30 mL per day for older children, adjusted based on the syrup concentration.^[13] Maximum daily limits should not be exceeded to prevent excessive sedation.^[13] Treatment is recommended for short-term use, up to 5-7 days for dry cough relief, and should not continue beyond the recommended period without medical advice to monitor for persistent symptoms or adverse effects.^[14]

Contraindications and precautions

Contraindications

Oxomemazine is contraindicated in patients with hypersensitivity to oxomemazine, other phenothiazines, or any excipients in the formulation, as this may lead to severe allergic reactions.^[13] It is also absolutely contraindicated in individuals with a history of agranulocytosis, due to the risk of hematological disorders associated with phenothiazine derivatives.^[13] Additionally, use is prohibited in cases of risk of narrow-angle glaucoma or urinary retention related to urethro-prostatic disorders, stemming from the drug's anticholinergic properties that can precipitate acute exacerbations.^[13][15] Severe hepatic impairment represents an absolute contraindication, as impaired metabolism can result in accumulation and heightened toxicity.^[7][16] It is also contraindicated in prostatic adenoma, paralytic ileus, pyloric stenosis, myasthenia gravis, status asthmaticus, and chronic obstructive pulmonary disease (COPD).^[1][4] Relative contraindications include prostatic hypertrophy, where the anticholinergic effects may worsen urinary outflow obstruction, though careful monitoring might allow limited use in select cases.^[15][7] Epilepsy is a relative contraindication owing to the potential for sedative effects to lower the seizure threshold in susceptible patients.^[15] Cardiovascular disease warrants relative avoidance, as the drug's sedative and anticholinergic actions can exacerbate hypotension or arrhythmias.^[15] Concurrent administration with monoamine oxidase (MAO) inhibitors is relatively contraindicated due to the risk of enhanced sedative and hypotensive effects from phenothiazine-MAOI interactions.^[17] The rationales for these contraindications are primarily linked to oxomemazine's pharmacological profile as a first-generation antihistamine with prominent anticholinergic and sedative properties; the former can intensify intraocular pressure in glaucoma or impede urinary flow in retention-prone states, while the latter may aggravate central nervous system conditions like epilepsy.^[7][13] In pediatric patients, oxomemazine is contraindicated for children under 2 years of age due to heightened risks of overdose-related convulsions, marked sedation, and potential respiratory depression, compounded by an association of phenothiazines with sudden infant death syndrome.^[13][18] This restriction underscores the need for alternative therapies in this age group to prevent severe adverse outcomes.

Special populations

Oxomemazine should be used with caution during pregnancy, particularly in the first trimester, where it is recommended to avoid administration due to limited data on fetal risks associated with antihistamines. In the third trimester, it may be prescribed only if clearly needed, as excessive doses could potentially harm the fetus, though specific teratogenic effects have not been well-established in human studies.^[19][20] Regarding lactation, the passage of oxomemazine into breast milk is not fully known, but due to the potential for causing sedation or paradoxical excitation in the infant, it is not recommended during breastfeeding.^[19][21] In elderly patients, oxomemazine requires caution owing to increased susceptibility to anticholinergic effects, such as orthostatic hypotension, sedation, and vertigo, which may elevate the risk of falls; dosing should start at the lowest effective level, often referring to adult guidelines but with close monitoring.^[21][1] For patients with renal or hepatic impairment, dose reduction or avoidance is advised in severe cases to prevent drug accumulation and prolonged effects, with regular monitoring recommended to adjust therapy as needed.^[21][1] Drug interactions are particularly relevant in special populations, where oxomemazine may potentiate CNS depression when combined with alcohol, opioids, barbiturates, or other sedatives, necessitating dose adjustments or avoidance in the elderly or those with comorbidities to mitigate excessive sedation. It can also enhance the effects or toxicity of sympathomimetics like epinephrine and interact additively with antihypertensives or other anticholinergics, requiring careful evaluation in patients with hepatic or renal issues.^[2][1]

Adverse effects

Common side effects

The most common side effect of oxomemazine is sedation or somnolence, which occurs frequently and is more pronounced at the beginning of treatment.^[22] This antihistamine-related effect arises due to its ability to cross the blood-brain barrier and is typically dose-dependent.^[1] Anticholinergic effects are also common, including dry mouth, constipation, and visual disturbances such as blurred vision or accommodation disorders.^[22] Urinary hesitancy or retention may occur, particularly in susceptible individuals, along with dizziness and balance disorders.^[22] Gastrointestinal disturbances like nausea and vomiting are reported less frequently but can contribute to overall discomfort.^[23] These adverse reactions are generally mild and transient, resolving upon discontinuation of the drug.^[22] Management includes advising patients to avoid activities requiring alertness, such as driving or operating machinery, especially during initial use. Rare allergic reactions, such as rash or urticaria, may occur but are not typical at therapeutic doses.^[22] Other rare effects include photosensitivity, orthostatic hypotension, tremors, motor incoordination, excitation (such as agitation or nervousness), mental confusion, and hallucinations (more common in the elderly).^[22][1] Serious rare adverse effects encompass hematological disorders like leukopenia, neutropenia, agranulocytosis, thrombocytopenia, and hemolytic anemia.^[22]

Overdose

Oxomemazine overdose primarily manifests through central nervous system (CNS) depression, ranging from severe drowsiness and disorientation to coma, along with convulsions that are particularly prominent in infants and young children.^[24][25] As a phenothiazine derivative, it also induces anticholinergic toxicity, including tachycardia, hyperthermia, and delirium, due to its blockade of muscarinic receptors.^[26] Acute risks encompass respiratory depression and hypotension from alpha-adrenergic blockade, with children under 2 years facing the highest vulnerability owing to their lower body weight and immature metabolic pathways.^[27][28] Management of oxomemazine overdose is entirely supportive, as no specific antidote exists. If ingestion occurred within 1-2 hours, activated charcoal (1 g/kg orally) should be administered to reduce absorption, followed by gastric lavage in cases of large ingestions under appropriate sedation.^[26] Intravenous fluids are essential to address hypotension, with vasopressors such as norepinephrine added if needed; benzodiazepines like lorazepam are used for convulsions, titrated to effect.^[27] Continuous ECG monitoring is critical to detect and manage QT interval prolongation, which may require magnesium sulfate infusion if torsades de pointes develops.^[26] All interventions should occur in a specialized medical setting with airway protection and mechanical ventilation readiness for respiratory compromise.^[24] Prognosis following prompt and aggressive supportive care is generally favorable, with most patients recovering fully within 24-48 hours and survival beyond 2 days indicating a positive outcome.^[27] Fatalities from isolated oxomemazine or phenothiazine overdoses remain rare, particularly with early intervention, though severe cases involving massive ingestion or delayed treatment have resulted in death due to refractory hypotension or arrhythmias.^[29][26]

Pharmacology

Pharmacodynamics

Oxomemazine exerts its primary therapeutic effects through competitive antagonism at histamine H1 receptors, a mechanism characteristic of first-generation antihistamines. By binding to these receptors, oxomemazine inhibits the agonist action of histamine, preventing downstream signaling via Gαq and Gβγ proteins. This blockade suppresses the activation of phospholipase C-β1, reducing the production of inositol 1,4,5-trisphosphate (IP₃) and 1,2-diacyl-sn-glycerol (DAG), which in turn limits calcium release from the endoplasmic reticulum through IP₃ receptors. Consequently, oxomemazine attenuates histamine-mediated allergic responses, including itching, inflammation, and the cough reflex triggered by peripheral irritation.^[30] As a phenothiazine derivative, oxomemazine demonstrates secondary anticholinergic activity, primarily through selective antagonism of muscarinic M1 receptors with high affinity (Kᵢ = 84 nM for high-affinity sites, compared to lower affinity at M3 receptors). This muscarinic blockade contributes to side effects such as sedation and dry mouth by interfering with acetylcholine signaling in both central and peripheral tissues. Additionally, oxomemazine exhibits antiserotoninergic properties, though these are less prominent in its overall profile. Unlike some phenothiazines, it shows reduced affinity for dopamine receptors, limiting antipsychotic-like effects even at higher doses.^[31][32][33] Due to its lipophilic structure, oxomemazine readily crosses the blood-brain barrier, enhancing central nervous system effects. This facilitates sedation via H1 receptor blockade in the brain and contributes to antitussive action by modulating cough reflex pathways in the brainstem, combining central suppression with peripheral histamine inhibition. Overall, oxomemazine lacks significant anti-inflammatory activity, focusing instead on symptomatic relief of allergic and cough-related conditions.^[34][32]

Pharmacokinetics

Oxomemazine is well absorbed from the gastrointestinal tract after oral administration. Peak plasma concentrations are achieved within 1 to 3 hours following dosing, consistent with the absorption profile of first-generation H1-receptor antagonists.^[35] The drug exhibits a high volume of distribution owing to its lipophilic properties, which facilitate extensive tissue penetration, including crossing the blood-brain barrier to exert central nervous system effects. Specific data on plasma protein binding are limited, though phenothiazine derivatives generally show moderate to high binding.^[13] Metabolism of oxomemazine occurs primarily in the liver through cytochrome P450 enzymes, involving oxidative pathways that produce multiple metabolites. These metabolites undergo phase II conjugation, and some may retain activity, contributing to the drug's extended duration of action beyond its plasma elimination phase.^[13] Excretion is predominantly renal, with metabolites accounting for the majority of elimination and only a small fraction of unchanged parent drug recovered in urine. The elimination half-life is variable but often prolonged, allowing for single daily dosing; sedative and antitussive effects may persist for 12 hours or longer due to active metabolites and tissue redistribution.^[13] Pharmacokinetic data remain limited overall, with much information extrapolated from the phenothiazine class; hepatic impairment may prolong clearance and increase accumulation risk, while information on other factors such as age or renal function is sparse.^[13][2]

Chemistry

Chemical properties

Oxomemazine has the molecular formula C$_{18}$18​H$_{22}$22​N$_{2}$2​O$_{2}$2​S and a molecular weight of 330.45 g/mol. Its IUPAC name is 3-(5,5-dioxido-10H-phenothiazin-10-yl)-N,N,2-trimethylpropan-1-amine.^[36] Its chemical structure consists of a tricyclic phenothiazine ring system featuring a sulfone group (5,5-dioxide) and a 3-(dimethylamino)-2-methylpropyl substituent attached to the nitrogen at position 10.^[37] Oxomemazine appears as a white to off-white crystalline powder.^[38][39] Key physical properties include a melting point of 115°C and a predicted boiling point of approximately 492°C.^[40] The compound exhibits low solubility in water but is soluble in ethanol (≥10.9 mg/mL), chloroform, and DMSO (≥33 mg/mL).^[41][42] Oxomemazine is sensitive to light, requiring storage in light-protected containers such as amber glass, and is also susceptible to oxidative degradation under stress conditions.^[21][43] It is classified as a phenothiazine derivative antihistamine, with the CAS number 3689-50-7 and the ATC code R06AD08.^[2]

Synthesis

The synthesis of oxomemazine, a phenothiazine derivative with a sulfone group at the 5-position (5,5-dioxide) and a 3-(dimethylamino)-2-methylpropyl substituent at the nitrogen, involves multiple steps starting from the phenothiazine core. The core ring system is typically formed by the condensation of diphenylamine with elemental sulfur under high-temperature conditions, often in the presence of a catalyst like iodine, yielding phenothiazine in high efficiency as a foundational intermediate for many derivatives.^[44] The key side chain is introduced at the 10-position (nitrogen) of phenothiazine through nucleophilic alkylation. Phenothiazine (9.6 g) is reacted with 1-chloro-2-methyl-3-(dimethylamino)propane hydrochloride in xylene (140 cc), using sodamide (2.77 g) as a base at 100°C under reflux for 2 hours to generate the phenothiazin-10-yl anion, followed by addition of the alkylating agent (90 cc of 0.61 N solution) over 50 minutes and further reflux for 20 hours. The mixture is cooled, treated with water (40 cc) and N-methanesulfonic acid (70 cc), and the aqueous layer is washed with ether. Basification with aqueous sodium hydroxide (d=1.33, 10 cc), extraction with ether, drying over potassium carbonate, and vacuum distillation (150-175°C at 0.3 mm Hg) afford the intermediate 3-(10-phenothiazinyl)-2-methyl-1-dimethylaminopropane as a base (12.6 g yield), which is converted to the hydrochloride salt (m.p. 216-217°C). This step achieves approximately 90% yield based on the starting phenothiazine.^[44] The final step involves selective oxidation of the sulfur atom in the intermediate to form the 5,5-dioxide. The intermediate base (11.9 g) is dissolved in glacial acetic acid (120 cc), treated with pure sulfuric acid (d=1.83, 0.5 cc), and then a mixture of glacial acetic acid (10 cc) and 38% hydrogen peroxide (8.5 cc) is added over 20 minutes (temperature rising to 25-35°C). The reaction is maintained at 60°C for 18 hours, cooled, diluted with water (150 cc), and neutralized with aqueous sodium hydroxide (d=1.33, 220 cc). Extraction with ethyl acetate (3 × 100 cc), evaporation, and recrystallization from heptane (150 cc) yield oxomemazine base (7.8 g, ~65% yield, m.p. 115°C). The hydrochloride salt is prepared in ethyl acetate and recrystallized from ethanol/isopropanol (m.p. 250°C).^[45] This multi-step process, originally detailed in U.S. patents assigned to the French company Société des Usines Chimiques Rhône-Poulenc (equivalent to 1960s French filings), represents the seminal industrial route with overall yields of 70-80% after optimization for purity. Modern variants emphasize impurity profiling during API production to minimize byproducts from over-oxidation or incomplete alkylation, though no stereochemical control is required, as oxomemazine is synthesized and used as a racemic mixture despite having a chiral center.^[45][44]

Society and culture

Brand names

Oxomemazine is commercially available under several brand names worldwide, primarily as a syrup formulation for cough suppression. The most prominent brand is Toplexil, which is often combined with guaifenesin as an expectorant and is marketed in various strengths such as 0.33 mg/mL oxomemazine in 150 mL bottles.^[46][2] Other common brands include Humex Toux Sèche, a single-ingredient syrup used for dry coughs, and Oxoril, typically formulated with guaifenesin for productive coughs in syrup form.^[46][47] Generic versions of oxomemazine are widely produced under names like Oxomémazine EG, Oxomémazine Biogaran, Oxomémazine Mylan, and Oxomémazine Sandoz, all in syrup formulations at concentrations around 1.65 mg/5 mL for oral administration.^[46] Combination products include Comtusi (with guaifenesin) and Oroxin (with guaifenesin), both syrups targeted at cough relief with allergic components.^[46] Tablets and injectables are rare and not commonly reported in major markets.^[46] Regional variations are notable, with Toplexil and its variants predominant in France, where it is available as a single agent or with additives like benzoic acid and paracetamol; in the Philippines, Toplexil is sold as a guaifenesin combination for over-the-counter use; and in Middle Eastern countries such as Kuwait, Lebanon, and the UAE, formulations like Rectoplexil (with guaifenesin and paracetamol) cater to local needs.^[46][20] Toplexil N, a variant without sugar, is marketed in Switzerland for similar indications.^[46] Key manufacturers include Sanofi-Aventis, which produces Toplexil and related brands across multiple countries, and generic producers such as EG Labo (for Oxomémazine EG in France), Biogaran (for Oxomémazine Biogaran), Sandoz, Mylan, and Teva Santé.^[46] In Indonesia, local firms like Combiphar (Comtusi) and Meprofarm (Oxoril) handle production of combination syrups.^[46]

Availability

Oxomemazine is approved for use in various countries, including France through the Agence Nationale de Sécurité du Médicament et des produits de santé (ANSM) as part of national procedures within the European Union framework. It is also approved in the Philippines by the Food and Drug Administration (FDA) and in Lebanon, where it appears on the national over-the-counter medicines list. However, it lacks approval from the U.S. Food and Drug Administration (FDA), classifying it as an unapproved antihistamine in that market.^[48][49] The drug was developed in France and first marketed under the brand name Toplexil for cough suppression. While it holds marketing authorizations in several regions, it has maintained investigational status in others, with limited expansion beyond initial approvals.^[2] Oxomemazine is available over-the-counter in many countries, such as France, Morocco, Egypt, Algeria, and the United Arab Emirates, primarily as a cough syrup for dry cough relief. In contrast, it requires a prescription in certain regions for allergy treatment, reflecting variations in regulatory classifications.^[50] A 2024 systematic review and meta-analysis concluded there is no evidence of clinical efficacy for oxomemazine in treating cough, raising questions about its therapeutic value and potentially leading to restricted use in approving jurisdictions.^[3] In its 2025 update, Prescrire International included oxomemazine in its list of drugs to avoid, citing an unfavorable harm-benefit balance across all indications.^[51] Despite this, the drug remains available via import in non-approving countries like the United States. No major recalls or withdrawals have been reported in its history of use, though its expansion has been limited due to concerns over sedation associated with first-generation antihistamines.^[46]